The present invention relates generally to optics and, more particularly, to a pair-wise symbol correlated high receiver sensitivity modulation format.
The present invention attempts to solve the problem of finding a modulation format that would have better receiver sensitivity than modulation formats that are currently being used. In current, ultra long-haul transmission systems, especially in legacy links, the modulation formats that are most frequently used are BPSK and QPSK. The current invention introduces a new modulation format still based on QPSK constellation however, with a better receiver sensitivity than both BPSK and QPSK. Having a better receiver sensitivity allows for extending the transmission distance at the same capacity. It also allows for reducing the total energy consumption.
In transmission systems, it is very critical to choose the right kind of spectral efficiency for a given distance to maximize the capacity. In general the maximum achievable distance drops dramatically as the capacity is increased by increasing spectral efficiency. The present invention has the spectral efficiency of 1.5 b/s per polarization, which is 50% higher than BPSK, while maintaining the same receiver sensitivity. At the same bit rate, the current modulation format requires a lower baud-rate, in this case it will deliver the same capacity at a better receiver sensitivity, and therefore at a longer distance.
Since for a given transmission distance there is an optimum spectral efficiency that maximize the capacity it is important to adjust the spectral efficiency depending on the required transmission distance. The easiest way to adjust spectral efficiency is to switch between different modulation formats and constellation sizes. Since transceiver architectures depend heavily on the kind of modulation formats, it is very costly to design and operate transceivers which can modulate and demodulate multiple constellations. Therefore it is very cost effective to design a configuration that can easily switch between different modulation formats. One advantage of the proposed modulation format is that it requires a simple modification of transceivers designed for standard constellations.
Others have attempted to solve the above problem by reducing capacity by using simplest of the constellations such as BPSK. While others have introduced modulation formats with increased receiver sensitivity such as polarization-switched QPSK. However, compared to polarization-switched QPSK, the new modulation format has the following advantages:                1. It can use the same DSP platform as the standard QPSK transceivers.        2. The two polarization tributaries are independent which makes the transceiver design simpler and more robust.        3. It is possible to use polarization bit-interleaving which provides better nonlinearity tolerance especially at legacy links with lower baud rates        4. It is possible to have this modulation format in single polarization a opposed to polarization-switching.        5. It is possible to extend the benefits of this new modulation format to higher constellations.        6. This method can be implemented in time domain between two symbols which may or may not be adjacent, or in frequency domain between subcarriers as in the case of OFDM where the correlated subcarriers may be adjacent in frequency or not.        
Accordingly, there is a need for finding a modulation format that would have better receiver sensitivity than modulation formats that are currently being used.